Inkjet printers, and thermal inkjet printers in particular, have come into widespread use in businesses and homes because of their low cost, high print quality, and color printing capability. The operation of such printers is relatively straightforward. In this regard, drops of a colored ink are emitted onto the print media such as paper or transparency film during a printing operation, in response to commands electronically transmitted to the printhead. These drops of ink combine on the print media to form the text and images perceived by the human eye. Inkjet printers may use a number of different ink colors. One or more printheads may be contained in a print cartridge, which may either contain the supply of ink for each printhead or be connected to an ink supply located off-cartridge. An inkjet printer frequently can accommodate two to four print cartridges. The cartridges typically are mounted side-by-side in a carriage which scans the cartridges back and forth within the printer in a forward and a rearward direction above the media during printing such that the cartridges move sequentially over given locations, called pixels, arranged in a row and column format on the media which is to be printed. Each print cartridge typically has an arrangement of printhead nozzles through which the ink is controllably ejected onto the print media, and thus a certain width of the media corresponding to the layout of the nozzles on the print cartridge, can be printed during each scan, forming a printed swath. The printer also has a print medium advance mechanism which moves the media relative to the printheads in a direction generally perpendicular to the movement of the carriage so that, by combining scans of the print cartridges back and forth across the media with the advance of the media relative to the printheads, ink can be deposited on the entire printable area of the media.
The quality of the printed output is a very important feature to purchasers of inkjet printers, and therefore manufacturers of inkjet printers pay a great deal of attention to providing a high level of print quality in their printers. Aberrations in the printhead nozzles can undesirably reduce print quality; such aberrations include, for example, not ejecting ink at all, ejecting an incorrect volume of ink in a drop, producing irregularly shaped drops with artifacts such as tails, or producing a spray of extraneous droplets in addition to the desired drop. Another common type of nozzle aberration is directionality error, also known as dot placement error, in which the drops of ink are not precisely printed in the intended locations on the print media. While sometimes printhead aberrations are due to the design of the printhead and thus are similar for all printheads of that particular type, other times the nozzle aberrations for a particular type of printhead differ from printhead to printhead. In addition, printhead aberrations can develop over time and with usage of the printhead; for instance, nozzles can become clogged or wear.
Nozzle aberrations frequently result in banding, or streaks of unprinted areas, on the printed output. To minimize banding due to nozzle aberrations (and coincidentally to also reduce the effect of printing defects resulting from having too much ink on the print medium at one time, such as bleeding of one color area into another and warping or wrinkling of the print media), most printers do not print all the required drops of all ink colors in all pixel locations in the swath in one single scan, or "pass", of the printheads across the media. Rather, multiple scans are used to deposit the full amount of ink on the media, with the media being advanced after each pass by only a portion of the height of the printed swath. In this way, areas of the media can be printed in on more than one pass. In a printer which uses such a "multipass" printing mode, only a fraction of the total drops of ink needed to completely print each section of the image is laid down in each row of the printed medium by any single pass; areas left unprinted are filled in by one or more later passes. When printing of a page is complete, every area of the print medium has typically been printed on by the same multiple number of passes. Because each pass uses a different nozzle to print a particular row of the image, multipass printing can compensate for nozzle defects. However, the typical multipass printmode in which all nozzles are enabled to deposit substantially the same amount of ink on each row of pixels is often insufficient to improve print quality to an acceptable level, particularly when some nozzles have worse errors than others, as in the case of nozzle aberrations as described above.
One approach to overcoming the shortcomings of multipass printing for compensating for nozzle aberrations is disclosed in commonly-assigned U.S. Pat. No. 5,124,720 filed Aug. 1, 1990 and issued to Schantz on Jun. 23, 1992 and titled "Fault-Tolerant Dot-Matrix Printing", which is hereby incorporated by reference in its entirety. This approach improves print quality by compensating for malfunctioning nozzles on a printhead-by-printhead basis. This method tests the printhead to identify inoperative printing elements, and then alters the scan path of the printhead so that properly functioning printing elements print where the inoperative printing elements normally would have. However, this method reduces the throughput (the number of pages that can be printed in a given unit of time, such as pages per minute) because it decreases the distance the paper is advanced after each pass of the printhead and thus increases the number of passes required to fully print a page.
Throughput is often just as important or more important to an inkjet printer purchaser as is print quality. Accordingly, there is still a need for an inkjet printer that minimizes print quality defects due to nozzle aberrations but without significantly reducing the throughput of the printer.